So, the Lost City trope doesn't encompass finding lost Middle Eastern, South American, European, Eastern Asian, Southern Asian, or (admittedly rarer) North American cities, city-states, temples, statues, and other remnants of a long dead empire?
So the Arabian stories about finding ruins in the sands of the city of glass destroyed by the sun in a fitful rage when his lover slept with someone in the city didn't help found the concept?
The Chinese stories of journeying across the lands and finding overgrown idols of unknown gods don't help?
The islander stories of searching for a continent the gods hid across the seas, it's beautiful lands untainted yet by man, these don't matter?
Stories from all corners of the globe of people looking for a city sunk by the gods, or of passageways in tombs, or the exploration of bizarre monuments for treasure, knowledge, or more treasure don't hold up? Just the ones about white dudes in Africa?
It doesn't stem from a racist idea, it stems from a human idea--we in the present are impossibly complex, while everyone in the past was primitive. Through study and diligence (something most people avoid like it has cooties), we can correct this misunderstanding in ourselves. Yes, age of exploration white dudes charting out Africa were racist, but when they looked at either nomadic or agrarian tribal societies versus the immense complexity of the eroding civilization, they saw no comparable way that the two groups could have been anything alike, ergo someone else built it. If you found intricate iron lacework and precision-crafted alloys near a group of people that hadn't the foggiest clue of metallurgy, you would conclude that someone else built it. It's really not that hard of a leap. Applying racist overtones onto it does nothing good.